docs: Fix broken image links in README.md

[KeithCu]

This PR fixes broken image links in the README.md file. The paths for three images (Opus46Resume.png, Sonnet46Spreadsheet.png, and Sonnet46ArchDiagram.jpg) have been updated to correctly point to the Showcase/ directory.

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PR created automatically by Jules for task 18421938903358972111 started by @KeithCu

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